Angle feedback is used in various electric drive applications. An elevator system may comprise such an electric drive, for example. A typical elevator system comprises an elevator car and a counterweight suspended on a cable that passes via a drive sheave. When the elevator is in use, an electric motor may drive an axle of the drive sheave in order to move the elevator car and the counterweight. When the elevator is not in use, a brake may be used to manually prevent rotation of the axle of the drive sheave.
The electric motor rotating the drive sheave may be an induction motor, for example. The rotational speed of the induction motor may be reduced to a level more suitable for an elevator by using a gear system. Alternatively, a slow-rotating, high-torque permanent-magnet synchronous motor may be used. Thus, the gear system can be left out and the motor can be attached directly to the axle of the drive sheave. In this manner, improvements in space requirements and cost-effectiveness of the elevator system can be achieved.
For safety reasons, and to guarantee a high-quality control, the control system a of permanent-magnet synchronous motor in an elevator application may be implemented as a torque control with an angle feedback from the rotor of the motor. However, after an angle sensor has been introduced into the system, the angle information produced by an angle sensor typically does not correspond with the actual rotor angle. Instead, there may be a constant angle offset between the sensed angle and the actual angle. This angle offset may have to be determined before the elevator can be used. After determining the angle offset, the offset may be added to the angle measured with the sensor during operation.
In general, an offset in a measurement of a rotor angle may be determined by applying a DC current to the motor and by assuming that the motor is able to align itself according to the applied current. The offset may then be determined from the difference between the angle of the applied DC current and the measured angle. However, this approach requires that the motor can turn freely. This may be problematic in elevator applications, for example, because mechanical installation of the motor may already have been done before the commissioning of a frequency converter.
Alternatively, a motor may be injected with a voltage stimulus and the actual rotor orientation may be determined from a current response induced by the stimulus. However, this approach requires a magnetically asymmetric (salient) permanent-magnet synchronous motor where the motor phase inductances are changing as a function of the rotor position. Thus, it is difficult to know beforehand if a certain motor is suitable for signal injection methods.